The present invention relates generally to methods for forming electrical interconnections. The present invention relates more particularly to a method for joining an integrated circuit or the like to a flexible circuit, wherein contact bumps that facilitate electrical interconnection have a profile or cross-sectional configuration which mitigates stresses therein while facilitating penetration of an oxide layer formed upon mating contact pads and wherein heat is applied so as to simultaneously cure a pre-applied epoxy underfill and fuse the contact bumps to the contact pads.
Methods for attaching integrated circuits and the like to printed wiring boards (PWBs) are well-known. Such methods enable the fabrication of various electronic subassemblies, such as motherboards and daughterboards for personal computers.
Contemporary methods for attaching integrated circuits to printed wiring boards include the use of various integrated circuit packaging technologies such as those associated with dual in-line package (DIP), plastic lead chip carrier (PLCC), ceramic pin grid array (CPGA), plastic quad flat pack (PQFP), quad flat pack (QFP), tape carrier package (TCP), ball grid array (BGA), thin small outline package gull-wing (TSOP), small outline package J-lead (SOJ), shrink small outline package gull-wing (SSOP) and plastic small outline package (PSOP).
According to DIP packaging technology, the two parallel rows of leads extending from the integrated circuit package pass through holes formed in the printed wiring board and are soldered into the holes. Optionally, a socket may be utilized.
Integrated circuits packaged according to PLCC and CPGA technologies typically require the use of a socket.
PQFP, QFP, TCP, BGA, TSOP, SOJ, SSOP and PSOP are examples of surface mount technology, wherein the packaged integrated circuit is attached directly to a printed wiring board, typically by such techniques as re-flow soldering and/or thermal compression.
For example, BGAs comprise a plurality of contact pads formed so as to define a 2-dimensional array upon the bottom surface of an integrated circuit package. Each electrical contact comprises a small ball of solder which facilitates interconnection to a complimentary array of flat electrical contact pads formed upon a printed wiring board. The small solder balls melt during reflow soldering to effect connection to a corresponding array of connectors formed upon the printed wiring board.
As the number of transistors formed upon a single integrated circuit increases, the ability to attach that integrated circuit to a printed wiring board or the like becomes more difficult. It is expected that the number of transistors formed upon a single integrated circuit will increase from its present number of approximately 80 million to approximately 100 million by the year 2000.
BGAs support high pin counts, so as to facilitate the use of integrated circuits having a larger number of devices formed thereon. By taking advantage of the comparatively large surface area on the bottom of an integrated circuit package, ball grid arrays provide for a comparatively large number of electrical interconnections between the integrated circuit and a printed wiring board.
Flexible circuitry for the communication of electrical signals between electrical components is well-known. Such flexible circuitry generally comprises a flexible dielectric substrate and a plurality of flexible conductive conduits formed thereon. For example, flexible circuitry may define wiring harnesses, transmission lines or part connectors. Flexible circuitry is frequently utilized to facilitate electrical interconnection between printed wiring boards or electronic assemblies.
It is frequently desirable to attach integrated circuits and the like to flexible circuits. Such direct connection of an integrated circuit to a flexible circuit eliminates the need for an intermediate printed wiring board, and thus results in a substantial cost savings. Thus, integrated circuits may be attached to flexible circuits so as to define electronic subassemblies which may either include or not include rigid printed wiring boards.
Generally, many of the same methods used to attach integrated circuits to rigid printed wiring boards may also be utilized to attach integrated circuits to flexible circuitry. Thus, sockets and/or contemporary surface mount techniques may generally be utilized to attach integrated circuits to flexible circuits.
Although such contemporary methods for attaching integrated circuits and the like to flexible circuitry have proven generally satisfactory, such contemporary methods do possess inherent deficiencies. For example, it is desirable to attach integrated circuits, whether packaged or not, to substrates such as flexible circuitry utilizing contact bumps which provide a stand-off distance between the integrated circuit and the flexible circuitry. This stand-off distance facilitates the accommodation of some degree of mismatch in thermal coefficient of expansion between the integrated circuit and the flexible circuit. That is, when either the integrated circuit or the flexible circuit contracts or expands more than the other, then the contact bumps accommodate some of the difference in such contraction or expansion by bending slightly.
However, such bending of the contact bumps may induce localized stresses which cause stress fractures. As those skilled in the art will appreciate, such stress fractures result in reduced conductivity through the contact bump, and may even result in complete discontinuity. Although it is possible to define contact bumps which are less likely to fracture due to localized stress buildup, such construction is contrary to achieving another desired objective of such contact bumps, which is to penetrate an oxide layer formed upon the contact pads to which the contact bumps mate.
As those skilled in the art will appreciate, an oxide layer frequently tends to form upon metallic contact pads. For example, the aluminum pads frequently used to define contact pads of integrated circuits, printed circuit boards and other electrical components tend to have a layer of aluminum oxide formed thereupon due to exposure to oxygen in the atmosphere. Such oxide layer tends to inhibit the formation of an adequate electrical contact with the electrical contact therebeneath. Thus, it is necessary to remove at least a portion of the oxide layer in order to facilitate desired electrical conduction with the electrical contact.
According to contemporary practice, contact bumps formed upon an integrated circuit mate with corresponding contact pads formed upon the flexible circuitry. In accomplishing such mating, the contact bumps must scratch, scrape, or otherwise penetrate an oxidation layer formed upon the contact pads. Failure of the contact bumps to penetrate such oxide layer results in an undesirably high resistance at the interface of the contact bump and the contact pad. This undesirable high resistance may result in failure of the device.
It is also desirable to enhance the mechanical attachment of an integrated circuit to flexible circuitry by utilizing an epoxy underfill. The epoxy underfill is typically wicked into place between the integrated circuit and the flexible circuit after attachment of the integrated circuit to the flexible circuit. However, according to contemporary practice, the use of such an epoxy underfill necessitates a separate heating in order to effect the curing thereof.
According to contemporary practice, one heating of the device is necessary in order to effect fusion of the contact bumps with the contact pads, so as to effect a desired electrical connection. Another heating of the device is necessary so as to effect curing of an epoxy underfill.
However, the polymers used in the fabrication of the flexible circuitry, as well as the integrated circuit itself, may be susceptible to undesirable degradation due to the excessive heating associated with two such separate heating processes. Further, as those skilled in the art will appreciate, the use of two such separate heating processes undesirably increases the costs associated with the fabrication of such devices.
In view of the foregoing, it is desirable to provide a method for attaching an integrated circuit or the like to flexible circuitry wherein the method mitigates localized stress buildup within the contact bumps utilized for such electrical interconnection, while maintaining a desired ability of the contact bumps to penetrate an oxide layer of corresponding contact pads, so as to assure adequate electrical contact therewith.
It is also desirable to provide a method which facilitates the use of an epoxy underfill while eliminating the need for an undesirable second heating procedure.
The present invention specifically addresses and alleviates the above-mentioned deficiencies associated with the prior art. More particularly, one aspect of the present invention comprises a method for attaching an integrated circuit to a flexible circuit by providing an integrated circuit having a plurality of contact pads formed upon a surface thereof and providing a flexible circuit having a plurality of corresponding contact bumps formed upon or integral to a surface thereof. The integrated circuit is attached to the flexible circuit by fusing at least some of the contact bumps of the flexible circuit to at least some of the corresponding contact pads of the integrated circuit.
The contact bumps have a shape which mitigates local stress buildup therein after attachment of the contact bumps to the contact pads. The shape of the contact bumps is such that penetration of an oxide layer of the contact pads is facilitated during such attachment of the integrated circuit to the flexible circuit.
Further, one aspect of the present invention comprises a method for applying heat so as to simultaneously fuse the contact bumps of one electrical device such as a flexible circuit to the contact pads of another electrical device such as an integrated circuit while simultaneously curing an epoxy underfill.
These, as well as other advantages of the present invention, will be more apparent from the following description and drawings. It is understood that changes in the specific structure shown and described may be made within the scope of the claims without departing from the spirit of the invention.